Handover is a vital part of any wireless or mobile communications network. A handover may be defined as the process of transferring an ongoing connection of a wireless device from one radio access network node (denoted as the serving radio access network node) to another radio access network node (denoted as the target radio access network node) in order to accomplish a seamless service over a large coverage area. The handover should be performed without any loss of data transmission to/from the wireless device and with as little interruption as possible for the wireless device.
To enable a handover, it is necessary to find a suitable target cell served by the target radio access network node, and to ensure that it is possible to sustain reliable communication to/from the wireless device in the target cell. Candidates for suitable target radio access network nodes (and/or target cells) are usually stored in so-called neighbor lists, which are stored at least at the serving radio access network node. To make sure that it is possible to sustain reliable communication to/from the wireless device in the target cell, the connection quality in the target cell needs to be estimated before the handover can be initiated.
The connection quality of the target cell is commonly estimated by measurements related to the wireless device. Downlink (DL, i.e., transmission from radio access network node to wireless device) and/or uplink (UL, i.e., transmission to radio access network node from wireless device) measurements may be considered. Relying solely on uplink measurements may not be sufficient, since the uplink connection quality can be different from the corresponding downlink connection quality. Therefore, handovers in cellular communications networks are commonly based on downlink measurements.
In existing cellular communications networks, all radio access network nodes (RANNs) continuously transmit pilot signals that wireless devices (WDs) in neighbor cells use to estimate the target cell quality. This is true in the Global System for Mobile Communications (GSM) where such pilot signals are transmitted on the broadcast control channel (BCCH), in the Universal Mobile Telecommunications System (UMTS) where such pilot signals are transmitted on the Common Pilot Channel (CPICH) and in the Long Term Evolution (LTE) telecommunications system where such pilot signals are transmitted as cell specific reference signals, as well as in Wi-Fi where such pilot signals are transmitted as beacons. This allows estimating the quality of neighbor cells with relatively good accuracy. The WDs perform measurements periodically and report the measurements to the network (i.e., the RANN). If it is detected that the serving cell quality approaches the candidate cell power, a more detailed measurement process or a handover procedure may be initiated. However, the signalling load from the RANN and the WD processing load depend on the number of candidate network nodes. Thus the signalling load from the RANN and the WD processing load may be significant for a large number of candidate network nodes.
Future cellular communications networks may use advanced antenna systems to a large extent. With such antennas, signals will be transmitted in narrow transmission beams to increase signal strength in some directions, and/or to reduce interference in other directions. When the antenna is used to increase coverage, handover may be carried out between transmission beams of the serving RANN or of the neighboring RANNs. The transmission beam through which the RANN is currently communicating with the WD is called the serving beam and the transmission beam it will hand over to, or switch to, is called the target beam. The potential target beams for which measurements are needed are called candidate beams.
Applying the principle of continuous transmission of pilot signals in all individual transmission beams in such a future cellular communications network may be convenient for WD measurements, and it may degrade the performance of the network. For example, continuous transmission of pilot signals in all individual transmission beams may consume resources available for data, and generate a lot of interference in neighboring cells, and higher power consumption of the RANNs.
US2013/0272263 discloses that time, frequency and spatial processing parameters for communications between a base station and a mobile station are selected by transmitting synchronization signals in multiple slices of a wireless transmission sector for the base station, and receiving feedback from the mobile station of at least one preferred slice of the multiple slices. In response to selection of one of the slices as an active slice for communications between the base station and the mobile station, reference signals are transmitted in the selected active slice using a corresponding selected precoder and/or codebook. The mobile station estimates and feeds back channel state information (CSI) based on those reference signals, and the CSI is then employed to determine communication parameters for communications between the base station and mobile station that are specific to the mobile station. The CSI-RS for different beams that are transmitted on the same time-frequency resources should be carefully chosen such that inter-beam interference is minimized. Additionally, different scrambling sequences or spreading sequences can be used for each beam such that inter-beam interference can be further suppressed.
US2013/0272263 relates to suppressing inter-beam interference at the WD when two CSI-RS are transmitted on the same time-frequency resource. US2013/0272263 is silent on the problem of resource usage and stacking of mobility signals during mobility measurement sessions initiated by the RANN. US2013/0272263 does not solve the issues of mobility between beams, of resource usage of mobility signals and of stacking of mobility signals.
Hence, there is a need for an improved mobility measurements session with minimized resource usage and reduced stacking of mobility signals.